a. Technical Field of the Invention
The present invention relates to laser velocimeters and, more particularly, an improvement in the output for transmitting optics for such devices.
b. Description of the Prior Art
Typical prior art laser velocimeter devices employ one or more pairs of single-mode optical fibers for coupling the laser beam to a transfer lens, one such transfer lens being provided for each beam. The beam then passes through a final lens (the laser velocimeter (LV) lens) which provides appropriate focussing thereof. More particularly, as explained in more detail below, the pairs of optical fibers are disposed with the axes thereof all parallel to each other at the output end and the light emerging from each fiber expands from the beam waist located at the output end of the fiber until the beam reaches the transfer lens. After passing through the transfer lens, the light narrows to a second beam waist (the transfer waist) and thereafter expands until striking the LV lens. Because the beams approaching the LV lens are mutually parallel, the beams cross each other at distance equal to one focal length of the LV lens beyond that lens. It is necessary in order to provide accurate LV measurements for all of the beams to have the waists thereof located at the common crossing point and this will occur if the transfer waists of the beams are properly located at a distance of one focal length of the LV lens before, i.e., behind or upstream of, the lens.
A serious disadvantage of the prior art is that if the laser beams are properly managed and arranged with respect to the size and location of the beam waist, the overall device is basically limited to small working distances (corresponding to the focal lengths of the LV lens) in that if longer working distances are desired, the overall configuration of the output optics is of impractical length. In other words, compact optical configurations featuring both proper management of the beam waists as described above and long working distances are not compatible with the techniques embodied in the prior art.
Considering other disadvantages of long working distances, it is noted that the alignment of the output ends of all fibers and the axes of all transfer lenses must be established and maintained with respect to the LV lens, and this places a great demand on the mechanical resolution and rigidity of the mounting devices for these optical elements when the optical elements are separated by long distances.
Further in this regard, it will be appreciated that a typical reason for using fibers in transmitting of optics is to enable the LV lens to be placed in scanning motion while the laser and the remainder of the beam-handling optics remain fixed. Such an arrangement significantly reduces the number of optical elements that must be subjected to extensive changes in the orientation of the scanning apparatus as well as to any attendant vibrations. However, a lengthy coupling path into the LV lens is an obvious disadvantage in such a scanning arrangement.
As is evident from the foregoing discussion, a long working path is a serious disadvantage for several reasons and the need for a long working path is tied to location of the transfer waist and hence to the requirement that a transfer lens be employed. Thus, it might at first appear that the problem could be solved by simply omitting the transfer lens at least in some special applications. In practice, however, it is unlikely that parameters such as the beam waist leaving the fiber, the required diameter of the measurement volume where the beams cross and the focal length of the LV lens could ever be made to be compatible without the provision of a transfer lens to provide adjustment of the waist size.
U.S. Pat. No. 3,895,872 (Dandliker et al) discloses an optical speed measuring device of background interest while the following patents are also of possible interest: U.S. Pat. No. 4,167,329 (Jelalian et al); U.S. Pat. No. 4,556,284 (Albersdoerfer et al); U.S. Pat. No. 4,705,367 (Eckbreth et al); and U.S. Pat. No. 4,753,421 (Deserns).